Display the version of the libary used for the documentation

[ThierryBerger]

Documentation is often lagging behind releases, leading to confusion for users:

• Update bevy_rapier2d to 0.29 + bevy_rapier3d to 0.29 #134
• Documentation doesn’t represents the behaviour #128

It would help to communicate to users which version of the library was documented, so they adapt their expectations.

Possible solutions

• Display the version of each library

That would make it clear which library version is documented, but the different versions are confusing.

• add an introduction in the getting started, to communicate library version (and underlying rapier (and parry?)... version)